Just inside the Capital Beltway in Fairfax County, the remnants of the Great Society can be found at Strawbridge Square, a complex where low-income families have been living since 1979 in 128 apartments and town houses built under the so-called "Section 8 program," a longtime mainstay of federal housing assistance programs.

"You would never know it is an assisted community. It looks just like it did the day it was finished," said Virginia S. Peters, head of a Methodist Church community group that helped plan and run the project. "Strawbridge Square is our pride and joy. We point to it as a model of community development in this nation."

Much of the responsibility for Strawbridge, Peters said, goes to the National Corporation for Housing Partnerships (NCHP), a little-known Washington company that has become the country's largest owner of low- and moderate-income housing since it was created by Congress in 1968. NCHP's companion, the National Housing Partnership, provided some of the start-up funds for the $3.6 million project, covered $500,000 in cost overruns as the project was being built, and continues to manage the Strawbridge Square units.

There are few Strawbridge Squares under construction anymore, however. Federal support for low-income housing, through such programs as Section 8, has withered under the Reagan administration. Many of the tax incentives for investing in low-income housing, furthermore, are being threatened by the tax-reform proposals now under debate on Capitol Hill.

The result has been difficult times for NCHP, as well as the other groups that traditionally have capitalized on federal programs to develop housing for poor- and middle-income people.

Though NCHP has in recent years begun to make money for the first time since it was created, its ability to fulfill its unique federal mandate of funneling private investment into low-income housing -- while turning a profit -- has been sorely put to the test.

But executives are hoping for a change in fortunes after a major corporate restructuring and capital infusion announced last week. The company said it received new capital of $36 million from subsidiaries of Weyerhaeuser Co., the Washington State forest-products firm, and Berkshire Hathaway Inc., the Omaha holding company headed by investment guru Warren E. Buffett.

The company also has been quietly mapping out new strategies for coping with the reduced federal involvement in housing. Some outsiders, who say NCHP has not been as aggressive as it should be, fear that the company will move progressively away from its congressional mandate and operate like a traditional real estate developer. Company executives deny that, but say they are simply taking pragmatic steps to ensure that they can carry out that mandate.

"There's no way to provide low- and moderate-income housing without subsidies and tax benefits, and Congress is eliminating them," said J. Roderick Heller, NCHP president and the driving force behind the recent recapitalization. "I don't want to suggest for a moment that we're not going to take risks. But we can not build only low-income housing without federal subsidies and with tax reform coming.

"I think it is inappropriate for people to think that NCHP can be a charitable institution. We have to become profitable and thriving," added Heller, who joined the firm last year.

Certainly both profitability and social progress were among the goals sought by the creators of NCHP in the late 1960s. The company was organized under a congressional charter in 1970, after a special commission established by President Lyndon Johnson had recommended the creation of private groups to encourage private industry to participate in producing housing for low- and moderate-income families.

Although tax-sheltered real estate syndications are now plentiful, the basic premise of the NCHP was unique at the time. Housing Investments Sought

The idea was to set up partnerships which would invest in low- and middle-income housing projects in which the principal economic benefit for investors would be tax losses. The grease for the system was various federal programs that subsidized the construction of this housing, although NCHP itself never has received any direct federal assistance.

For a time, the system worked reasonably well, and NCHP carved out a small but important niche in the industry, according to real estate experts.

With $42 million in initial investment provided by 270 companies and organizations -- some of whom simply wrote the contribution off as charity, according to Heller -- the housing partnership has become the general partner in nearly 500 development projects around the country. Over the years, the company has raised about $540 million from 6,500 investors in these projects.

These projects contain almost 80,000 units, mostly apartments, while the company's property management subsidiary also has some 50,000 units under its direct management. Two dozen of NCHP's projects dot the metropolitan Washington landscape, including Strawbridge Square, the Foxchase complex in Alexandria, and Congress Park, a government-owned housing project in Anacostia that was redeveloped by the partnership in the early 1980s.

"NCHP has basically fulfilled its role," said Carl A. S. Coan, a Washington attorney who served at the Department of Housing and Urban Development (HUD) when the company was created. "It has been a very substantial producer or participant in the production of low-income housing."

As he quickly added, though, "There are certain points beyond which even the angels of the world cannot go without federal assistance." Indeed, over the course of the 1980s, the basic meal tickets of the company were wiped out.

One was the federal subsidies that made the production of low-income housing possible. Between 1981 and 1985, HUD saw its funds for housing assistance cut from $32 billion a year to $10 billion, the biggest reduction in any domestic program.

In addition, the Reagan administration's initial tax-reform plan, and the subsequent congressional plans, propose to eliminate many of the traditional tax incentives for investing in low- and moderate-income housing.

Under the tax bill passed late last year by the House of Representatives, for instance, the amount of tax deductions that investors can take for depreciation on low-income housing was sharply reduced. Meanwhile, the use of tax-exempt bonds -- which have financed many low-income housing projects -- would be curtailed as well.

As if these changes, which would undercut the company's traditional benefits for investors, were not enough, NCHP also has had to grapple with its own internal financial difficulties. The company was burned in the early 1980s by a number of expensive and ultimately unsuccessful ventures into single-family housing deals, while it also suffered a severe cash-flow squeeze at the beginning of last year. Reported Profit in 1985

The company did report profits of $5.7 million in 1985 -- largely on the strength of its property management affiliate -- but Heller noted that much of the original capital raised for the firm has been eaten up over the years. As a result, he has sought since last year to raise additional capital for NCHP while trying to devise a new business strategy for the company.

One of the men he turned to was Warren Buffett, who has made millions on investments in The Washington Post Co., Geico Corp. and other companies. Buffett said he was put in touch with Heller through Lloyd H. Cutler, the former Carter administration official whose law firm Heller once worked for before coming to NCHP. He apparently liked what he saw.

"Right now, it is in a somewhat unpredictable phase, but that doesn't mean it's valueless," said Buffett, whose company is putting about $24 million into a new holding company for NCHP. "They've got a record of not making the local investor disappointed in them . . . . I think that a record of doing what you say in this field is a pretty good asset," he added.

Buffett also said that senior managers' investment of their own money in the company -- at his insistence -- would be an important performance incentive. Heller himself has purchased $740,000 worth of the new holding-company stock.

With Buffett on board, Heller was able to raise about $12 million from Weyerhaeuser Co. He and the other executives also simplified NCHP's corporate structure, and they say it should be easier now to raise even more capital in the future -- perhaps with a public stock offering at some point. NCHP Evaluating Its Mission

For now, however, management at NCHP is trying hard to figure out how to fulfill its mission -- and make money -- within the narrow parameters shaped by current federal housing policy. Indeed, under the circmustances, they insist there is little they will be able to do to produce new housing for very low-income persons, a belief that disturbs some tenant and housing activists.

One lawyer who has represented some community-development groups said NCHP has been conservative in its approach to difficult housing projects in the inner cities while being reluctant to deal with local nonprofit groups. "They are acting very much like a traditional syndicator which is trying to maximize profits for private investors," said this source, who would not comment if he were named.

An NCHP proposal, subsequently dropped, to renovate the Abingdon Apartments in Alexandria is red meat for critics, who charged that the project would have displaced the very low-income tenants it was supposed to help. NCHP denied the charges, but Mark W. Looney, a housing official in Alexandria, said the company "seems to operate on its own with syndications rather than try to cooperate with the community or tenant groups."

Looney said that NCHP's overall role has been constructive, however. "Before a lot of these federal programs were eliminated, they had a very positive role in helping provide housing for low- and moderate-income families in Alexandria," he said.

As a result of the withdrawals of these subsidies, Heller said NCHP will try increasingly to tap state and local funds for development. But he also said NCHP will rely more on outside capital for individual projects than on its own equity as it has in the past; meanwhile, it will seek to beef up revenue through fee income -- for instance, through its property management services.

Heller also said the company hopes to tap new sources of capital, citing pension funds and foreign investors as promising possibilities. Heller noted that he has a tentative commitment from one pension fund to provide $100 million in funding for various NCHP projects.

"We have enormous responsibilities," said Heller. "We now own 80,000 units. Last year, we invested $5 million in advances to those projects for operating deficits. If we don't do that those projects will be ruined. They'll be urban slums or suburban slums.

"One can view what was done back in 1968 and '70 as a compact between private capital and Congress," Heller continued. "Congress said it would provide federal housing subsidies and a tax system which facilitated investment to the private investors. We in turn would provide the capital. Congress is breaking that compact . . . . We, however, are going to remain committed to that compact."